Directional overcurrent relays are widely used for the protection of power distribution systems such as radial and ring subtransmission systems and other distribution systems. These relays have a functionality enabling them to determine a fault direction. Here, a fault can mean an overcurrent, for example, from a short circuit. Further, the fault direction is in most cases binary information, indicating whether the fault is a forward fault or a backward fault. Here, in a power line connecting an upstream power source to a downstream power distribution system portion (with the normal power direction from upstream to downstream), the forward direction is downstream of the relay, and the backward, or reverse, direction is upstream of the relay.
In smart grids, decentralized or distributed units can feed power into the grid or consume power from the grid. Thus, in smart grids the power flow direction can change with time. In this situation, “forward” and “reverse” can still be defined as above with respect to the current power flow, so that e.g. the forward direction will change if the power flow is reversed.
The fault direction can be an indicator at which side of a measurement location the fault has occurred. In the above example, there are two directions, forward and backward. If the measurement location is at a node of the power network having more than two sides, there may be more than just a forward or a backward direction. For example, for a node to which one backward line portion and two forward line portions are connected, the fault direction can include the cases “forward-1”, “forward-2”, and “backward”.
The directional information provides a more detailed information about the location at which a fault has occurred. This information can be used to deactivate a smaller portion of the power distribution system in the case of a fault. For example, a known ring-main feeder, e.g. for domestic supplies has breakers at its T-junctions. If there is a fault in any of the lines of this known ring-main feeder, the whole line section can be interrupted. This situation can be improved when more detailed fault directional information is obtained. To address this situation, directional overcurrent relays can be installed in the line along with breaking switches. With such a relay-switch system, a reference voltage measurement allows computing the fault current and its direction. The directional information can then be used to disconnect only the appropriate section, rather than the whole line.
Known directional overcurrent relays rely on a reference voltage phasor, also known as “voltage polarization”, for estimating the direction of the fault. When a fault occurs, the fault current has a characteristic phase angle relative to the voltage phasor, the phase angle depending on the fault direction. The fault direction is determined by comparing the current phasor (complex current value whose real part is the actual AC current) to a reference voltage phasor (industrially termed as ‘voltage polarization’) measured at a measurement location on the power line. This specifies measurement of both current and voltage. This approach becomes unreliable when the fault is in close proximity to the relay, because in this case, the relay is almost grounded by the short circuit (industrially termed as ‘close-in faults’).
Further, overcurrent relays including a voltage measurement unit are expensive. Since they have to be used in large number for the above arrangement, this is a major cost factor.
Eissa M. M. discloses in “Evaluation of New Current Directional Protection Technique Using Field Data”, IEEE Transactions on Power Delivery, IEEE Service Center, New York, N.Y., US, vol. 20, no. 2, (1 Apr. 2005), pages 566-572, XP011129251, ISSN:0885-8977 a current polarized directional element technique to determine the fault direction on a transmission line using fault data recorded in field. To determine the fault direction a sum of plain current signals is evaluated. It is concluded that for a forward fault, the absolute sum of the fault current signal and the polarized current signal are higher than the absolute value of the fault current signal. On the other hand it is concluded that for a reverse fault, the absolute sum of the fault current signal and the polarized current signal are lower than the absolute value of the fault current signal. The obtained absolute sum which does not contain any phase information is further used in the proposed technique to identify directionality and therefore compared to a threshold value. In other words, no accumulation of values containing a phase difference is used.